Alasdair Wilkins

There's an evolutionary reason why people cry when they cut onions - it's the body's way of keeping us away from dangerous chemicals. And it's such a useful adaptation that every living animal on Earth possesses it.

The mechanics of why we cry when we cut onions have been understood for a while. Onions contain chemicals that are released when the knife cuts through the cell walls, causing a reaction that creates sulfuric acid. This substance can irritate or even damage human tissue. To defend against this, the protein TRPA1, found throughout the body, sends pain signals to the nervous signal. This pain sensation is meant to warn people to get away from the gas, and those that choose to keep cutting will usually start to cry.

What scientists did not have a good sense of until recently was the evolutionary background of this response. Biologists at Brandeis University have now discovered that this exact same mechanism can be found in fruit flies, and it operates in precisely the same way. This suggests that the TRPA1 protein can be traced all the way back to our common ancestor hundreds of millions of years ago, as biologist Paul Garrity explains:

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"While many aspects of other chemical senses like taste and smell have been independently invented multiple times over the course of animal evolution, the chemical sense that detects these reactive compounds is different. It uses a detector we have inherited in largely unaltered form from an organism that lived a half-billion years ago, an organism that is not only our ancestor, but the ancestor of every vertebrate and invertebrate alive today."

To put that in perspective, this response dates back to a time before land animals even existed, when the most advanced organism was probably the sea star. Garrity and his team suggest that there was an evolutionary shift during the Cambrian Period (543 to 490 million years ago) in which a strain of organisms emerged that possessed the TRPA1 protein. These organisms had a huge reproductive advantage, in that they could detect toxic chemicals and use this early warning system to avoid potentially fatal situations.

Further research on TRPA1 will largely focus on its practical applications. There is hope that finding ways to remove the TRPA1 response in humans will help treat pain and inflammation, while heightening the TRPA1 response in mosquitoes and similar organisms might reduce their propensity to pick up and spread diseases.